Let me jump in with the first word of advise. I know a lot more will be forthcoming from others, but...very important....All types of maples are prone to a lethal plant disease known as Verticillium Wilt. It lives in the soil and enters a maple when the roots, especially the surface feeder roots, have been cut which is what tilling does. Best to not till under your maples. Clean up the debris and litter beneath them. Spread topsoil lightly so as not to smother tree roots.

its tough to establish and maintain plants of any size with the competition of the roots from the trees.....you have to be careful about how much you do under the trees as far as tilling or adding topsoil,you'll kill the trees......try perennials that tolerate dry shade, hosta is a no brainer for this area.....your best bet is to establish at least some large patches of groundcover like pachysandra,periwinkle, etc

What the other two posters said. Most of the feeder roots of trees are in the top 12 inches or so of soil. Disturbing them is not a good idea. Birches are fairly sensitive souls, should be treated for Bronze Birch Borer by a professional, and can up and die for no apparent reason without warning. Treat them gently.

Adding more than a few inches of soil over the roots of established trees is also a very bad idea.

The Hostas and groundcovers are a good idea, also check out the Shade forum for more suggestions for plants that will do well in your conditions. Treat the trees gently when inserting plants in the root area, which can extend well beyond the width of the tree canopy. Three inches or so of organic mulch will help keep down weeds and retain moisture until your plants fill in. If you don't plan on a fairly dense planting under there, plan on major weeding chores.

I've seen alot of people deal with a stand of small trees by covering a large area surrounding it with mulch. Likely, you would have to renew every year, but it beats looking at weeds.

We're two zones apart, so I won't venture any particular plant suggestions, but have you visited wooded/shade gardens in your area? I believe you have plenty of options for your stands, and I don't think anything compares with strolling through some areas that have already been nicely planted for getting inspiration.

I recently visited a private woodland garden that was breathtaking, and last year strolled through the woodland gardens at our state university's arboretum; both gave me loads of ideas, and I know I haven't even scratched the surface of such gardens yet!

If you have a Master Gardener program, Extension Service, or local garden club in your area, give them a call and see if they can recommend a few places for you to visit.

As others have already mentioned, the health of the existing trees is important, so don't hesitate to ask for - and then follow - sound advice on designing and installing your shade gardens.

Absolutely what everyone else said, and also... have you thought about a small seating area? I don't know if you need a seating area, but I can absolutely see a small area paved with informally set flagstones with a bench, somewhere in the middle of the group. It would look great!

Shade-loving azaleas and a small garden bench would be very attractive. The azaleas will grow quickly, so don't overdo it. Less competition for the soil space. :)

Unfortunately, maples and birches are among the most difficult trees to plant anything under. They have surface roots that suck nutrients and moisture out of the soil. Certainly, never till and don't pile on new soil. It will suffocate the roots if you put down too much, and the roots will want to grow into the new soil anyhow.

You don't say where you are in zone 5b, but if you're in the east, look for the most drought resistant shade plants you can find. Liriope may work, also Lamium 'Herman's Pride', certain Epimediums, and Sarcococca hookeriana var. humilis (a broad leafed, slowly spreading evergreen groundcover for shade). Pulmonaria will suffer from the moisture competition.

Bulbs will work in your situation; just dig holes and plant them with a good bulb food and feed them yearly. Daffodils are good and so are crocus, chionodoxa, scillas. Tulips may work, but look for perennializing varieties (most tulips are one shot deals).